Active mufflers work with anti-noise, which is actively generated by at least one loudspeaker and can be used by phase cancellation to attenuate, i.e., suppress certain frequencies, e.g., to reduce noise emission into the environment by an exhaust gas system of an internal combustion engine.
Active mufflers are known, for example, from U.S. Pat. No. 5,600,106, for U.S. Pat. No. 5,233,137, from EP 1 055 804, for U.S. Pat. No. 5,336,856, from EP 0 755 045, from U.S. Pat. No. 5,343,533, from EP 1 627 996, from U.S. Pat. No. 4,177,874, from U.S. Pat. No. 5,229,556, from U.S. Pat. No. 5,432,857, from EP 0 674 097, from U.S. Pat. No. 5,619,020, from EP 0 916 817, from DE 197 51 596, from EP 0 373 188.
One problem with active mufflers in exhaust gas systems of internal combustion engines is thermal stress on the loudspeaker due to the high exhaust gas temperatures. Whereas traditional loudspeakers are able to operate in a range of 100° C. to 200° C., the exhaust gas temperatures usually amount to 400° C. to 700° C. Therefore, an increased effort is required to prevent overheating of the loudspeaker. Furthermore, when using active mufflers in exhaust gas systems for internal combustion engines, the condensate generated during operation may be problematical. The exhaust gases of an internal combustion engine may have a relatively high moisture content, so that condensate develops on surfaces that are cool in relation to the exhaust, e.g., cooler than 100° C. In particular, condensate may thus develop on the diaphragm of a loudspeaker. Condensate collecting on the loudspeaker can create a burden on the diaphragm, substantially impairing the effect of the loudspeaker and causing it to freeze in the presence of frost, which may ultimately lead to destruction of the loudspeaker. Furthermore, a loudspeaker requires a comparatively large rear volume, in particular when comparatively high sound pressure levels are to be generated in a low-frequency range. However, accommodating such a comparatively large rear volume may lead to substantial design space problems, especially in exhaust gas systems for motor vehicles.